Generally, a magnetic-type position detector detects a change in the position of a detection object relative to a reference part. The magnetic-type position detector may utilize a magnetic flux generation unit such as a magnet. For example, a position sensing device disclosed in a patent document 1 (i.e., Japanese Patent Laid-Open No. JP-A-H08-292004) is configured to form a closed magnetic circuit having two magnets and two magnetic flux transmission units that are provided as the reference member. In such a structure, the two magnets or their magnetic poles are bound at in-between positions of end portions of the two magnetic flux transmission units, which may be more specifically described as a position between one of two longitudinal ends of respective magnetic flux transmission units that extend side-by-side. A gap space between the two magnetic flux transmission units has a spill magnetic flux flowing from one unit to the other. A flux density detection unit is configured to move together with the detection object within the gap between the two magnetic flux transmission parts and to output a detection signal according to the magnetic flux passing therethrough. Thereby, the position sensing device detects a position of a detection object relative to the reference member based on the signal outputted from the flux density detection unit.
In the position sensing device of the patent document 1, the dimension of the magnets along a magnetized direction is set to be the same as the width of the gap space between the two magnetic flux transmission units. Therefore, when the gap space between the two magnetic flux transmission units is large, the magnet must also have a large size, which may result in an increase in the manufacturing cost of the position sensing device. In such a case, two end portions (i.e., both sides) of the magnetic flux transmission unit may be modified to have a specific shape, for example, for reducing the size of the magnets that are put in between those end portions. When the size of each of the magnets along the magnetized direction is smaller, the cost of manufacturing the device is reduced. However, a complex shape of the end portions of the two magnetic flux transmission units may require use of complex material bending in order to form the magnetic flux transmission unit, or may require use of additional manufacturing processes such as forging, cutting, and the like, which may further increase the manufacturing cost.
Further, in the position sensing device of the patent document 1, the signal outputted from the flux density detection unit becomes large, when the flux density detection unit comes close to the magnet. Therefore, especially at the proximity of the magnetic, a linearity of the output signal from the flux density detection unit may deteriorate.
In the position sensing device of the patent document 1, the two identical magnets having the same size, material, etc. are disposed on both ends of the two magnetic flux transmission units with their magnetic pole directions reversed from each other. Therefore, at the center of the gap space between the two magnetic flux transmission units, a magnetic flux flow direction is reversed. In other words, a minimum position where an absolute value of the flux density falls to the minimum occurs at the center of the movable range of the flux density detection unit and the movable range of the detection object (i.e., at the center of the gap space between the two magnetic flux transmission units).
Generally, at the “minimum position where an absolute value of the flux density falls to the minimum” in the movable range of the flux density detection unit, the magnetic flux of the magnetic flux generation unit least affected by a temperature coefficient factor, which favors a resistance of the position sensing device to the temperature change. In other words, at the “minimum position where an absolute value of the flux density falls to the minimum” the position detection accuracy of the detection object is improved relative to the other positions. Therefore, in the position sensing device of the patent document 1, the position detection accuracy is high at the center position of the gap space between the two magnetic flux transmission units, and the position detection accuracy is low at positions other than the center position of the gap space, for example, near the ends of the gap space, etc. In other words, in the position sensing device in the patent document 1, positions having a high position detection accuracy regardless of the temperature change are limited to only the center of the movable range of the detection object. That is, if the position sensing device requires a high position detection accuracy near both ends of the movable range of the detection object, the position sensing device in the patent document 1 may be not suitable for such applications.
The position sensing device in the patent document 1 may still be usable if the shape of the magnetic flux transmission unit, for example, is modified to have a gradually-changing width of the magnetic flux transmission unit, or to have a gradually-changing gap space width between the two magnetic flux transmission units along the longitudinal direction of the gap space, for the purpose of improving linearity of the output signal from the flux density detection unit, or for the purpose of moving the high position detection accuracy position to a position other than the center position of the detection object movable range regardless of the temperature. However, in such a case, the complex shape of the magnetic flux transmission unit may result in increased manufacturing costs.